Reinstate pltr_data tests for NULL that were removed by revision 10895.  It turns out those tests are essential for the python and tcl bindings to work properly.
Reinstate pltr_data tests for NULL that were removed by revision 10895.  It
turns out those tests are essential for the python and tcl bindings to
work properly.

svn path=/trunk/; revision=10959

FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot